Why doesn't my unwrapped model appear in the UV Editor?
To unwrap UV's, you mark any seams you want, select the faces you want to unwrap, press U and select one of the unwrap options while in edit mode.

Only selected faces (or those surrounded by selected vertices/edges) will be unwrapped. Only selected faces will show up in the UV editor.

Also UV's aren't displayed in the UV/image editor if the Render Result is displayed in the UV/Image Editor. You need to select an image or click the X to close the render result to see UV's.